Media provider form + Subtitles in audio / video

[didoda]

When an audio or a video has captions (association Captions should be activated for the Audio or Video model), subtitles can be activated in audio|video player (html5 provides track tag to handle that).

This changes the "Media" section, to show provider fields with the player.

This refactorizes the stream item data into a media-item component and provides subtitles for audio|video, when available.

An example of video file (no provider) with captions:

An example of remote video (provider youtube):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.64%. Comparing base (62e2b3c) to head (3d1df79).
Report is 28 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1211      +/-   ##
============================================
- Coverage     99.65%   99.64%   -0.01%     
+ Complexity     1326     1314      -12     
============================================
  Files            98       97       -1     
  Lines          5205     5120      -85     
============================================
- Hits           5187     5102      -85     
  Misses           18       18              

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